1. Field of the Invention
This invention relates to a pulse-width modulation circuit for the amplification of analog signals such as an audio signal, and more particularly to a self-oscillation type pulse-width modulation circuit requiring no carrier signal source.
2. Description of the Prior Art
FIG. 1 shows a conventional direct feedback type pulse-width modulation circuit in which a pulse-width modulated signal is directly fed back to an input thereof. This pulse-width modulation circuit comprises an operational amplifier 1 whose non-inverting input terminal is supplied with an input signal Vi. An inverting input terminal of the operational amplifier 1 is connected to ground through a resistor 2 (value Ra), and a capacitor 3 (value C) for integration is connected between the inverting input terminal and output terminal of the operational amplifier 1. This pulse-width modulation circuit further comprises a hysteresis comparator circuit 4 and a feedback resistor 5 (value Rb) connected between an output terminal of the hysteresis comparator circuit 4 and the inverting input terminal of the operational amplifier 1. The hysteresis comparator circuit 4 comprises an operational amplifier 6, resistor 7 (value R1) and 8 (value R2) and is supplied with an output voltage V1 of the operational amplifier 1 as an input signal. An output signal V0 of this hysteresis comparator circuit 4 appears at an output terminal of the operational amplifier 6.
With this construction, during a period when the output signal V0 is +E (See FIG. 2) the voltage V1 is decreased at a constant slope of -K1 since the capacitor 3 is charged with a current determined by the following formula. EQU (E-Vi)/Rb-Vi/Ra
And at the time when the voltage V1 drops beyond a negative threshold -(R1/R2)E of the hysteresis comparator circuit 4, the output signal V0 goes to low (-E). Then when the output signal V0 is -E, the voltage V1 is increased at a constant slope of +K2 since the capacitor 3 is discharged with a current determined by the following formula. EQU (Vi+E)/Rb+Vi/Ra
And at the time when the voltage V1 rises beyond a positive threshold +(R1/R2)E of the comparator circuit 4, the output signal V0 goes high (+E). And thereafter, the above-mentioned operation is repeated.
With this pulse-width modulation circuit, the inclinations -K1 and +K2 of the voltage V1 vary in accordance with the voltage of the input signal Vi, so that a duty factor D of the output signal V0 linearly varies with the voltage of the input signal Vi while a frequency F of the output signal V0 quadratically decreases with the increase of absolute voltage of the input signal Vi. This pulse-width modulation circuit is therefore suitable for an audio amplifier.
However, it is essential that the output voltage V1 of the operational amplifier 1 should have enough amplitude to exceed both positive and negative threshold levels of the hysteresis comparator circuit 4, since this pulse-width modulation circuit employs hysteresis characteristics of the comparator circuit 4 to obtain self-oscillation conditions. This means that a part of gain of the operational amplifier 1 necessary to obtain a voltage corresponding to the voltage between the thresholds, that is to say, a hysteresis width of the comparator circuit 4, does not contribute to the effective amplification. It will be appreciated that the hysteresis comparator circuit 4 has a dead band correspondind to the hysteresis width. An open-loop gain of this pulse-width modulation circuit is decreased by an amount equal to the gain loss of the operational amplifier 1, so that the distortion reduction effected by the negative feedback is also decreased by the gain loss. Further it is impossible for this pulse-width modulation circuit to lessen solely the hysteresis width to overcome the above-mentioned drawbacks, because the self-oscillation conditions are determined by the hysteresis width.